Those ubiquitous “pop-up” and “pop-under” Web ads could become obsolete after Microsoft announced it would phase them out of its MSN Internet units by summer.

The software giant says research showed that customers were growing increasingly dissatisfied with the ads that appear in new windows when certain MSN Web sites are accessed.

As a result, Microsoft will no longer sell that ad format to advertisers. The ban will start shortly on MSN’s media properties in the United Kingdom, the Nordic countries and Belgium, then extend globally.